The invention disclosed herein relates generally to a tool for inspecting, refurbishing or repairing welds in a nuclear reactor and, particularly, refurbishing welds around a core shroud of a boiling water nuclear reactor (BWR).
A BWR typically includes a generally cylindrical chamber that forms a wall of a pressure vessel and a core shroud within the pressure vessel. The core shroud contains the nuclear reactor core, and both are immersed in water/coolant that floods and flows through the pressure vessel.
Between the pressure vessel and the core shroud is an annular gap that extends around the shroud and has a width of about eighteen inches (0.45 m). The gap is filled with coolant/moderator that is circulated through the core by jet pumps. The gap provides an area for assemblies of jet pumps, piping and other components that are arranged on the outside of the core shroud and direct the coolant/moderator into the reactor core. The jet pump assemblies, piping and other components are typically deep down in the gap and submerged in coolant and moderator fluid. In addition, the gap provides a pathway for tools and instruments used to inspect and repair welds on the core shroud and the jet pump assemblies, piping and other components arranged in the gap.
Welds on the core shroud are subject to corrosion and cracking because they are immersed in coolant and water and because of other environmental conditions. The welds are periodically inspected refurbished and repaired. To reach the weld, the core shroud is opened and tools are lowered by cables down through the gap between the pressure vessel and the core shroud. It is particularly, difficult to lower tools and inspection instruments to welds and components deep down in the weld due to the narrow gap, the coolant/moderator fluid and the jet pump assemblies and other components in the gap. Specific tools, sensors and carriages to support the tools and sensors are needed to access welds and other components deep in the gap. One difficulty is securing a carriage to the core shroud or jet pump assembly while the tool or sensor on the carriage performs work on the core shroud or other component in the gap. For example, if the tool is a wire brush that refurbishes the weld by smoothing a weld the wire brush must be forcibly applied to the weld and the carriage must be firmly fixed to the shroud as the wire brush smoothes the weld. Accordingly, there is a long felt need for tools and instruments to access the welds in the gap between the shroud and pressure vessel.